Ophiolites are claimed to be
pieces of ocean crust and upper
mantle that have been thrust up onto
continental crust and are now found
especially in mountains and along
continental margins.
28–30 Numerous
ophiolites outcrop extensively in
the mountains from the Alps eastward into the Himalayas.
31 An
ideal ophiolite suite consists from
bottom to top of peridotite, gabbro,
sheeted dikes, basalt with pillow
lavas, and sedimentary rocks. The
peridotite is an upper mantle rock,
while the remainder of the sequence
is considered ocean crustal layers.
However, there are parts of this
vertical sequence commonly missing,
except for the upper mantle rocks.
For instance, the sheeted dike complex and the sedimentary rocks are
often missing. The basalt can also vary
from thin to absent. So, ophiolites are
mainly identified by upper mantle
rocks, and they may not necessarily
represent ancient ocean crust since
one or more of the oceanic upper
crustal components are missing.

Ophiolites can be over 10 km thick and
sometimes of large geographical scale,
such as the impressive arc-shaped
Oman ophiolite that is about 150 km
wide and 550 km long (figure 6).
32, 33

The origin of ophiolites has long
been a subject of controversy.
34 A
favoured hypothesis is that ocean crust
was generated at mid-ocean ridges
(MORs); spread out from the MORs;
and, after colliding with continents,
was forced up and over the continental crust, in some cases for possibly
hundreds of kilometres. Ophiolites
sometimes possess high temperature
metamorphic rocks at their bases,
35
the grade of metamorphism decreasing
downward below the base, indicating heating from sliding friction.
36
However, most ophiolites are now
believed to have something to do with
subduction zones, in which an oceanic